Antiserum to estradiol-17 beta and the antiestrogen CI - 628 will be tested for their individual effects on implantation of the hamster. The effect of indomethacin, and H1-and H2-histamine receptor blockers will also be studied for their effect on implantation, spacing of the conceptuses and the fate of the subsequent gestation. In an attempt to correlate the endocrinological status with the morphology of the developing hamster, blood will be drawn for radioimmunoassays of estradiol, estrone and progesterone. Hamsters will be bled for this purpose on days 3, and 4 of the cycle (P.O. equals Day 1), the first seven days of gestation (covering implantation), the time of initiation of the chorioallantoic placenta on day 9, the attainment of the definitive placenta on day 12, and parturition, pre during and post. Each animal used will be autopsied immediately for assessment of physiological state.